This invention relates to rotational molding systems in which there are multiple mold halves that are repetitively opened and closed. More particularly, it relates to a system for automatically opening, closing, clamping, and unclamping such molds.
Various types of rotational molding systems are well known. For example, U.S. Pat. No. 5,306,564 and 5,529,481 describe mold systems where two or more pairs of mold halves can simultaneously be brought together to form mold cavities in which molded items are cast.
Meltable particulate (usually a plastic such as polyethylene) is placed inside each cavity, each mold is closed and placed in a furnace where it is heated (or the molds are otherwise heated), and each mold is rotated and sometimes also rocked.
The particulate melts and adheres to the internal surface of each cavity. After cooling, each mold is opened and the molded items are removed. This type of molding is particularly well suited for items that have large hollows in them (e.g. large plastic toys; plastic gasoline containers).
In high production facilities problems can arise in connection with the opening and closing of such molds. As taught in U.S. Pat. No. 5,529,481, a system is needed to hold the mold halves tightly together during the molding process. This is particularly important because of the range of movement of the molds during molding, and because if the mold halves are not tightly held together plastic can seep into the joint between mold halves, thereby creating unwanted flashing (and wasting plastic).
To achieve a tight joint the art had previously used various bolting systems. However, such bolting systems were labor intensive and time consuming to use. In some cases, workers experienced repetitive strain injuries due to the need for repetitive turning of bolts and/or nuts.
Moreover, if the bolts/nuts were not evenly tightened there could be uneven wear at the mold joint and/or instances of seepage at the joint. While patents such as U.S. Pat. No. 5,306,564 disclosed a system to reduce the number of bolts required, that system still required some bolts. Even with that system there was some concern regarding achieving even clamping pressure on opposite sides of the molds, and when comparing one mold pair to another.
In U.S. Pat. No. 5,869,109 a system was provided to simultaneously clamp and unclamp a pair of molds. While this system was a significant improvement, it still required manual clamping and locking of the molds.
It can therefore be seen that a need exists for an improved multi-mold closure system that reduces still further the manual operations of the molding process.